Use of flash storage devices has been rapidly increasing over the years because they are portable and they have small physical size and large storage capacity. Flash storage devices come in a variety of designs. Some storage devices are regarded as “removable” which means that a user can move them from one host device to another or replace one storage device with another. Other storage devices are regarded as “embedded” which means that they cannot and are not intended to be removed by the user from a host device with which they operate. For various reasons, manufacturers of embedded storage devices preload user data into storage devices before they are incorporated into the hosts. In general, user data is preloaded into the storage device before a host is distributed to an end user with the storage device embedded in it. Global Positioning System (“GPS”) maps, music files, video files, video games, and the like, are examples of user data.
Memory cells that are operated as Single-Bit per Cell (“SBC”) cells are known for having higher data-retention durability than memory cells that are operated as Multi-Bit per Cell (“MBC”) cells. However, user data is traditionally stored in memory cells that are operated as MBC cells because user data are typically large (e.g., in the order of hundreds of megabytes to gigabytes), and storing them compactly in MBC cells saves storage space.
Typically, a storage device is embedded in a host device by using a reflow soldering process. MBC cells with the preloaded user data are susceptible to possible effects of the heat generated from the reflow soldering process and the data in them may be altered as a result, particularly because of the relatively small error margins that exist between the different binary states of the memory cells (i.e., smaller error margins than those between binary states in SBC cells). In other words, excess heat that is generated as a result of the reflow process decreases the threshold voltage levels of the memory cells, thus causing them to unintentionally transition from designated binary states to other (i.e., erroneous) binary states.
There is therefore a need to address the problem of reflow-induced discharge of electric charge in memory cells when a storage device is embedded in a host device. There is also a need to preload user data to MBC cells and, at the same time, to guarantee the integrity of the user data.